1. Field of Invention
The present invention relates to a method of producing a magnesium (Mg)-based alloy. More particularly, the present invention relates to a method of synthesizing magnesium-cobalt (Co) pentahydride (Mg2CoH5).
2. Description of Related Art
A Mg-based alloy is generally referred to as an alloy material which is mainly composed of Mg. With the features of lightweight and high hydrogen absorption capacity, the Mg-based alloy has high potential to be applied on a fuel cell system.
Using a Mg-based hydride (MgH2) as an example, although the MgH2 hydride has a high hydrogen absorption capacity up to 7.6 weight percent (wt %), yet the MgH2 hydride has a high hydrogen desorption temperature and is difficult to be formed, thus having limited applications.
Generally speaking, adding another metal, such as nickel (Ni), Co or iron (Fe), etc., into the Mg-based alloy to form magnesium-nickel quadhydride (Mg2NiH4), a magnesium-cobalt pentahydride (Mg2CoH5) or a magnesium-ferrum hexahydride (Mg2FeH6) may improve the property of hydrogen absorption-desorption kinetics for the Mg-based alloy, i.e. the Mg-based alloy may have a better hydrogen absorption-desorption efficiency than pure magnesium, wherein the magnesium-cobalt pentahydride (Mg2CoH5) has a preferable hydrogen absorption property.
Since pure cobalt is not a hydrogen absorption element, the magnesium-cobalt pentahydride (Mg2CoH5) may have a greater hydrogen absorption amount if containing less pure cobalt. However, at one hand, it is quite difficult to synthesize the magnesium-cobalt pentahydride (Mg2CoH5) through a metastable Mg2Co. At first, according to the Mg—Co binary phase diagram, MgCo2 is the only existing stable Mg—Co compound, and the metastable Mg2Co cannot be synthesized via a conventional smelting process.
On the other hand, the magnesium-cobalt pentahydride (Mg2CoH5) obtained by the conventional process cannot overcome the problem of leaving residual pure cobalt. Briefly speaking, in the conventional process, magnesium metal power and cobalt metal powder with a mole ratio of 2:1 are ball-milled for 100 hours to more than 200 hours, and then are sintered at high temperature and high pressure in an environment full of hydrogen to form the magnesium-cobalt pentahydride (Mg2CoH5). After XRD (X-ray Diffraction) analysis, a cobalt diffraction peak still exists in the magnesium-cobalt pentahydride (Mg2CoH5) obtained, i.e. complete reaction still cannot be achieved even after the ball-milling step has been performed for 200 hours.
In view of the foregoing, there is a need to provide a method of synthesizing magnesium-cobalt pentahydride for overcoming the problem of poor hydrogen absorption-desorption rate caused by the conventional ball milling/sintering or reactive mechanical alloying.